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A Born-von Kármán rigid-ion, lattice-dynamical model has been applied to diopside with the use of empirical potentials derived by fitting the vibrational frequencies of a group of nesosilicates and oxides (including quartz). The Raman and infrared spectra are satisfactorily interpreted by these calculations; there is also good agreement with anisotropic atomic displacement parameters (ADPs) derived from accurate crystal-structure refinement at various temperatures. The agreement with theoretical and experimental data of independent origin confirms the physical meaning of both experimental and calculated ADPs; consequently, the correlation tensors between the displacement of different atoms, which can be evaluated in this way and cannot be obtained from Bragg diffraction measurements, are also reliable, thereby allowing bond-length correction for libration in the most general case. As for other oxides and silicates, the zero-point motion contribution amounts to about 35% of the total at room temperature. Hirshfeld’s rigid-bond test applied to the SiO 4  groups is in agreement with our calculations. The values of thermodynamic functions such as the specific heat and entropy at different temperatures are also satisfactorily reproduced, although a rigid-ion model has been used and the empirical potentials were not specifically fitted to the properties of the substance under study: For this purpose, a simple treatment of anharmonic expansion is proposed. Therefore, the possibility of transferring empirical potentials to widely different groups of minerals and that of developing a routine application to derive accurate values for various properties of pure minerals at various temperatures, starting only from crystal data, are confirmed.

Lattice-dynamical evaluation of atomic displacement parameters of minerals and its implications: The example of diopside